Answer:
The third side is sqrt(5)
Explanation:
This is a right triangle so we can use the Pythagorean theorem
a^2+b^2 = c^2 where a and b are the legs and c is the hypotenuse
We know the legs are 1 and 2 and we are looking for the hypotenuse.
1^2+2^2 = c^2
1+4 = c^2
5 =c^2
Take the square root of each side
sqrt(5) = sqrt(c^2)
sqrt(5) =c
The third side is sqrt(5)